Slowblow fuse burns after power's been removed

[DarpMalone]

Hi all. This is something I've never seen before.

I've just built a basic linear PS with a 450 VA, 65vct transformer and 20,000uF per rail.... The transformer is being switched by an optical relay on the primary.

When I apply power, the amp powers up and everything seems to work as expect until I turn the amp back off. This is when the 5A slow blow fuse burns out.

Any idea why this is happening?

 

[trobbins]

What do you mean by 'optical relay' - an SSR (solid state relay)?

Perhaps try some temporary changes and see what happens:
- disconnect circuitry to the secondary winding.
- use a normal power AC switch instead of the optical relay contact.

 

[epicyclic]

Can you visibly see this happening or is it that the fuse will only handle one switch on surge and the next time you switch the amp on it blows instantaneously .

 

[Speedskater]

Remember to use a 'light bulb/dim bulb' tester when troubleshooting.

 

[DUG]

... switched by an optical relay on the primary.

...

Part number of optical relay?

 

[DarpMalone]

Hi all. Thanks for your replies.

It's a solid state relay, sorry. (Electrol S2181)

I did use a bulb tester in my initial testing and all seemed fine (just a very dim glow after the initial surge wile the caps charge.

The strange thing is, it doesn't blow my fuse every time. It's happened 3x though. When I take the relay out of the equation everything works normally.

Sent from my Nexus 7 using Tapatalk

 

[trobbins]

Unless there is something else quirky happening then focus on the drive circuit for the SSR, or swap the SSR in case it is faulty or underrated for the application.

It may be possible to use an alternative control signal (eg. battery or plugpack) to switch the control input of the SSR.

A faulty SSR, or a noisy control signal, may be causing the transformer to draw much higher current levels as a result of in-rush/magnetising transient.

There is also a chance that the peak inrush current is exceeding the 10A SSR rating. This can be alleviated by using a higher current rated SRR, or inserting an NTC for some soft-start effect.

 

[DarpMalone]

Well after removing the SSR from the circuit everything seems fine. Maybe the way that I've implemented it is wrong?
I'm borrowing 12v from a speaker protection circuit (when I switch on the power it powers up the speaker protection board, sending 12v to the DC control input of the SSR, slightly more than the max that relay's looking for. Could this be related?

I have a soft start module that I was planning to use elsewhere. I guess I'll try that.

Thanks for your help.

Sent from my HTC Amaze 4G using Tapatalk

 

[DUG]

It's a solid state relay, ... (Electrol S2181)

...

Can't seem to find a data sheet.

Could you provide a link to one or provide the pdf, please?

 

[DarpMalone]

Yeah, strangely I can't find it either :-(

 

[DUG]

Picture?

 

[DarpMalone]

I'm not home to take a pic right now but this is the one
http://www.ohmelectronicsinc.com/ebay/store/061808/061808-01.jpg

Sent from my HTC Amaze 4G using Tapatalk

 

[funk1980]

Well after removing the SSR from the circuit everything seems fine. Maybe the way that I've implemented it is wrong?
I'm borrowing 12v from a speaker protection circuit (when I switch on the power it powers up the speaker protection board, sending 12v to the DC control input of the SSR, slightly more than the max that relay's looking for. Could this be related?

I have a soft start module that I was planning to use elsewhere. I guess I'll try that.

Thanks for your help.

Sent from my HTC Amaze 4G using Tapatalk

A SSR is basically an optocoupler (LED into light dependent device) controlling a triac. The specified voltage of 10V will never be seen by the triac side. You just have to make sure there's not going to much current to the SSR's input, or you might kill the LED. The output part might be faulty, causing the blown fuse.